That the commercial airliners in the US had been hijacked, per se, was not the problem. The two arms of the government most responsible for air defense of the US homeland, the FAA and the North American Aerospace Defense Command (NORAD), had both contemplated such a situation, according to a report of the National Commission on Terrorist Attacks Upon the United States released Thursday.

Protocols for the FAA to obtain military assistance from NORAD in such a situation already existed. But the protocols were based on several mistaken assumptions. The most likely scenario, they felt, was for an airliner to be hijacked overseas, and then head for US airspace, allowing plenty of time to formulate a response.

Thus standard operating procedure called for multiple levels of notification and approval of actions at the highest levels of government.

Nor did agency officials contemplate that the military assistance might entail shooting down the aircraft. Protocols called for fighter aircraft to trail hijacked planes at a discreet distance of five miles, for example, and to simply monitor progress of the captured aircraft. "On the morning of 9/11, the existing protocol was unsuited in every respect for what was about to happen," says the commission report. "What ensued was a hurried attempt to create an improvised defense by officials who had never encountered or trained against the situation they faced."

The chaos of that morning was underlined Thursday in public testimony before the commission by Gen. Richard Myers, chairman of the Joint Chiefs of Staff. Reports of car bombs in Washington and mistaken assumptions that other aircraft had also been hijacked created an atmosphere of confusion. "We fought many phantoms that day," said General Myers.

As an indication of the chaos swirling through the halls of government, Vice President Dick Cheney at one point announced to an interagency teleconference that he believed the US military had already shot down several airliners, according to the commission report.

At another point, the pilot of an F-16 launched from Langley Air Force Base in Virginia looked down on the burning Pentagon and surmised that the damage had been caused by a Russian sneak attack.

Some counterterrorism officials within the US national security structure immediately knew that the attacks could only be a coordinated Al Qaeda assault. But that was not a scenario that had been at the top of the FAA's and NORAD's list of security worries. "There was a lot of circulating evidence that airplanes were going to be used by Al Qaeda, as early as the 1990s, but that they were going to be used this way would have taken such a leap of imagination by planners, I think in many ways it has to be forgiven," says Juliette Kayyem, a homeland security specialist at Harvard University.

The 9/11 report released Thursday methodically tracked the progress of the four hijacked airliners and matched that against a timeline of government response. The air defense of America began at 8:38 a.m. on that fateful day, notes the report, when the FAA's Boston center air-traffic-control facility notified NORAD's Northeast Air Defense Sector that American Airlines Flight 11 had been hijacked. Perhaps understandably, the first question asked by the Air Force personnel who received the call was whether the situation was "real-world, or exercise?"

At that point, American Airlines Flight 11 was only nine minutes away from hitting the World Trade Center. Yet the first F-16s from Otis Air Force Base in Massachusetts would not get off the ground until seven minutes after that impact. Even then, they would not be given any target information. Lacking that, they were simply vectored into military airspace off the Long Island coast.

After that, the US defense response simply remained many steps behind the rapidly unfolding chain of events. The FAA controller in charge of United Airlines Flight 175, the second plane hijacked, did not notice its suspicious behavior for some minutes - in part because he was also responsible the first plane hijacked and was still searching for it.

The third plane taken, American Airlines Flight 77, flew undetected for 36 minutes, in part because FAA controllers did not realize it had turned around and was heading east. Even the best efforts would not have been able to protect Washington against the fourth plane, United 93, concluded the commission. Time was just too short. "NORAD officials have maintained that they would have intercepted and shot down United 93. We are not so sure," concludes the commission report.